The efficacy and stability against Listeria monocytogenes of nisin and lysozyme encapsulated in phospholipid liposomes was evaluated. Antimicrobial-containing liposomes were prepared by hydrating dried lipids with buffer containing nisin, nisin plus the fluorescence probe calcein, or calcein and lysozyme. Mixtures were then centrifuged and sonicated, and encapsulated liposomes were collected using size-exclusion chromatography. Antimicrobial concentration in liposomes was determined by bicinchoninic acid assay prior to determination of antimicrobial activity against strains of L. monocytogenes. When nisin was encapsulated in liposomes, protein concentrations of 0.39, 0.27, and 0.23 mg/ml for phosphatidylcholine (PC), PC-cholesterol (7:3), and PC–phosphatidylglycerol (PG)–cholesterol (5:2:3), respectively, were obtained. Encapsulation of nisin with calcein yielded protein concentrations of 0.35, 0.39, and 0.28 mg/ml for PC, PC-cholesterol, and PC-PG-cholesterol, respectively. Encapsulation of calcein with lysozyme resulted in protein concentrationsof 0.43, 0.26, and 0.19 mg/ml for PC, PC-cholesterol, and PC-PG-cholesterol, respectively. Encapsulated nisin in 100% PC and PC-cholesterol liposomes inhibited bacterial growth by >2 log CFU/ml compared with free nisin. Growth inhibition with liposomal lysozyme was strain dependent, with greater inhibition observed for strains 310 and Scott A with PC-cholesterol and PC-PG-cholesterol liposomes. Inhibition of L. monocytogenes indicated the potential of liposomes to serve as delivery vehicles for antimicrobials in foods while improving stability of antimicrobials.

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